how to stretch part of object
Ok, I found something, but surly is not the right way to do it, also having some issue with this method:


Select one part of the side 
SHIFT + D to duplicate
Delete the primary part of the side you just duplicate
in edit mode select the vertex you want to stretch.
press E then right click to cancel.
on the submenu add the value you want (4cm in Z in my case)
move it back together.


In my case it's not really smooth